1. Field of the Invention
The present invention relates to a magnetoresistive device in which a sense current is allowed to flow in a direction perpendicular to film plane for detecting an external magnetic field as well as to a magnetic head and a magnetic recording-reproducing apparatus.
2. Description of the Related Art
Conventionally, read-out of information recorded in a magnetic recording medium has been performed with a reproduce magnetic head including a coil, which is moved relative to the recording medium so as to detect a voltage induced in the coil by electromagnetic induction. Then, a magnetoresistive device (hereinafter referred to as MR device) has been developed and has been used in, for example, a magnetic field sensor and a magnetic head (hereinafter referred to as MR head) mounted to a magnetic recording-reproducing apparatus such as a hard disk drive.
In recent years, a marked progress has been achieved in miniaturization and increase in capacity of the magnetic recording medium, which lowers a relative speed between the reproduce magnetic head and the magnetic recording medium in reading out information. Therefore, a high expectation is put on the MR head that permits a high output even if the relative speed noted above is low.
Under the circumstances, it is reported in, for example, Phys. Rev. Lett., 61, 2474 (1988) and Phys. Rev. Lett., 64, 2304 (1999), that a so-called artificial lattice film produces a gigantic magnetoresistance effect. The artificial lattice film is a multilayered film prepared by alternately laminating ferromagnetic metal films and nonmagnetic metal films, such as Fe/Cr and Fe/Cu, under predetermined conditions such that adjacent ferromagnetic metal films are antiferromagnetically coupled with each other. However, the artificial lattice film requires a high magnetic field for saturating the magnetization and, thus, is not adapted for use as a film material for the MR head.
On the other hand, some examples are reported that a multilayered film of a sandwich structure of [ferromagnetic layer/nonmagnetic layer/ferromagnetic layer] produces a large magnetoresistance effect even if the ferromagnetic layers are not antiferromagnetically coupled with each other. To be more specific, an exchange bias magnetic field is applied to one of the two ferromagnetic layers having the nonmagnetic layer interposed therebetween so as to fix the magnetization, and the magnetization of the other ferromagnetic layer is reversed by an external magnetic field such as a signal magnetic field. As a result, a large magnetoresistance effect can be obtained by changing a relative angle of directions of the magnetization of the two ferromagnetic layers arranged to have the nonmagnetic layer sandwiched therebetween. The multilayered film of this type is called a spin valve, as reported in, for example, Phys. Rev., B45, 806 (1992) and J. Appl. Phys., 69, 4774 (1981). The spin valve, which permits saturating the magnetization with a low magnetic field, is adapted for use in the MR head and has already been put to a practical use. However, a coefficient of magnetoresistance change (GMR coefficient) is at most about 20% for the spin valve, making it necessary to develop an MR device exhibiting a higher GMR coefficient.
In the conventional MR device, the sense current is allowed to flow within the film plane (Current in Plane: CIP). On the other hand, it is reported in, for example, J. Phys. Condens. Matter., 11, 5717 (1999) that, if the sense current is allowed to flow in a direction perpendicular to the film plane (Current Perpendicular to Plane: CPP), it is possible to obtain a GMR coefficient about 10 times as much as that for CIP mode and, thus, a GMR coefficient of 100% is not impossible. However, in the spin valve structure, the total thickness of the layer dependent on spin is very small, and the number of interfaces is small, with the result that the resistance itself is rendered low in the case of current perpendicular to plane and an absolute value of output is rendered small. If the current perpendicular to plane is applied to the spin valve of the structure employed in the conventional CIP mode, the absolute value of output AΔR per 1 μm2 is small, i.e., about 0.5 mΩμm2, in the case where the thickness of the pinned layer and the free layer is 5 nm and, thus, a further increase in the output is required.
In order to obtain a high output in the spin valve structure, it is important to increase the resistance value of the portion involved in the spin dependent conduction so as to increase change in the resistance.